Secure documents have been produced in the past by letterpress, offset, and intaglio printing processes, including such processes adapted for embedding hidden information or security images. Document embedded security features deter counterfeiting of valuable papers, important records, and financial instruments such as checks, currency, so that original documents can be verified and unauthorized copies of these documents can be readily distinguished from the originals.
The printing of such original documents can be carried out either in black-and-white (B&W) or in color; and if in color, in spot color, colored backgrounds, or multicolor printing. Multiple colors are often preferred for original documents to enhance aesthetic value and ease of recognition, as well as to protect the documents from copying by conventional means. Printing processes for printing valuable originals, whether in B&W or in color, include intaglio (e.g., gravure), letterpress, and offset printing, among others. These and the other processes mentioned in this application are very well known in the art and will not be discussed in great detail.
One approach to deterring counterfeiting or other forms of unauthorized reproduction of originals includes embedding “latent images” containing covert information or other indicia (e.g., security codes, graphics, or information such as amounts of money and bank account numbers) within the original documents. The latent images, which are hidden within a visually integrated setting, are intended to be practically invisible to the naked eye under normal viewing conditions. Such settings for hiding the latent images range from simple background tints to the remainders of composite images within which the latent images are hidden. However, the latent images are otherwise distinguishable through the use of a visual aid, such as a magnifier or reader. Upon reproduction, such as by scanning or copying of the originals, the latent images can be arranged to either disappear or become more pronounced. That is, either the absence of the latent image or the visible presence of the latent image in a copy can be construed as an indication of a non-original document.
The latent images can be produced by embedding one line-screen pattern within another line-screen pattern. Each of the line-screen patterns can be formed by an array of print elements, such as dots, dashes, line segments, or other elemental shapes, in an arrangement of systematically spaced lines. The two line-screen patterns can be formed so as to be indistinguishable from one another to the naked eye, but can differ in some respect that can be optically exploited for viewing the latent image. For example, the two line-screen patterns can be printed in the same color and at the same print density but can differ in orientation or line frequency.
A first line-screen pattern can be printed in defined areas having one or more shapes for forming the latent image. A second line-screen pattern can be printed throughout an adjoining or an enlarged area forming a visually integrated setting, such as a complementary background or a remaining portion of a composite image. Each of the two line patterns can be formed by a plurality of line patterns in different colors so long as the multiple line patterns that form the latent image match the color and overall appearance of the one or more line patterns that form the visually integrated setting.
Other developments for purposes of providing document protection are disclosed in the patent literature, as for example, in U.S. Pat. No. 5,018,767 issued May 28, 1991; U.S. Pat. No. 5,193,853 issued Mar. 16, 1993; and U.S. Pat. No. 3,675,948 issued Jul. 11, 1972; and U.S. Pat. No. 4,143,967 issued Mar. 13, 1979, all to Ralph C. Wicker. All of these patents, which are hereby incorporated by reference, disclose various means for providing methods and products to enable copies of documents to be distinguished from the originals, as for example, by a “large dot-small dot pattern”, a “close line-spaced pattern”, and images or indicia which are screen printed at minutely varied spaces and/or angles on the originals and are intended to produce a highly visible moire pattern effect on the unauthorized copies. In this specification, the words “print”, “printed” and “printing” are used to refer to the making of an original document by any of a number of known printing means, including transferring images from one source to another, typically a paper medium, using a transfer agent such as ink or toner. The words “copy” and “copying” to refer to making copies from an original printed document.
Special viewing devices, referred to as “authentication viewers” or “document viewers”, can be used to distinguish latent security images from the visually integrated setting in which the images are embedded. Such viewers can incorporate tints or matching patterns that differentially affect the appearance of the line screen patterns that make up the security images and their integrated settings. For example, the viewers can incorporate patterns that complement one or the other line screen patterns, creating the necessary contrast to render the security images visible.
Different line screen patterns require authentication viewers with different matching patterns. Thus, the use of a range of line-screen patterns for hiding security images requires the use of multiple viewers, which must be matched to the original printed documents. This can add additional cost and complexity and tends to limit the number of different screen patterns available for use. Missing viewers or attempts to authenticate documents in different locations can delay or otherwise frustrate the authentication of the documents.